I'm running a Comp Cams custom grind camshaft with 605/622 (1.6) lift. Currently I'm running Comp Cams 1.6 S/A Pro Magnum rockers arms. The car is down right now for some head work, and I'm going to go with NSA rockers, and thinking about going with the Pro Magnum 1.52 rockers to bring my lift down a bit. Any idea if I will loose or gain HP by reducing the lift, 605/622 seems a little extreme for my heads?

Also, I'm running CNC Cylinder Heads ported LT1 heads. Flow is ~270/199 at .600 lift.


Thanks
-Mike

 

You will gain nothing (other than perhaps extend spring life a bit) and may lose a fairly substantial amount of HP.

Rich

 

In my 95 vette I'm running more intake lift than you and about the same on the exhaust with no-drama (Its my DD) however, my valvetrain was built for it. (Springs are Pac 1220s)

 

As a general "rule of thumb" 1.6 RRs provide about .030" to .040" lift at the valve over 1.5s.

WD

 

Also, valve lift is a little misleading. Engines often make more hp when the valve lift is greater than the choke point. The reason is that the higher lift cam will usually have more duration at each point along the lift curve. The so-called "area under the (lift) curve" will be greater. IOW, the valve will spend more time at or above the highest flow lift points. If valves opened and closed instantaneously, then the "ideal" lift profile would instantly open and close the valves to the point of maximum flow at the ideal crank angle.

I wrote "ideal" in quotes because I am told that there is some benefit to closing the exhaust valve relatively slowly under certain circumstances. This is counterintuitive and I have never seen it adequately explained. But some smart people have said it, so it may well be true. In F1, where valves are very light and opened and closed using electical/pneumatic devices I imagine they have a lot of control over valve events. It would certainly be interesting to know about how they program the opening and closing rates, but technical info in F1 is guarded like national security.

In this specific example, going to 1.52:1 rockers will make the lift 0.575/.591" and I bet the hp loss would be on the order of 3-5%. For sure, a lower rocker ratio not only decreases the maximum lift but does also decrease the duration at intermediate lift points. In reverse, that's the reason a higher rocker ratio almost always leads to substantial power increases. It makes the valve motion more aggressive even if the cam lobes are chosen so that the max lift is the same. The trend in OHV racing motors is towards higher and higher ratios for the reason that lobe acceleration rates are limited by the diameter of the lifter. So, once you max these out the only way to get quicker valve action is by increasing the rocker ratio. This has the added benefit that hihger ratios "multiply" the force of the spring on the head side of the valve train. Kind of like getting something for nothing, in many ways. The only problem with higher and higher rocker ratios concerns the geometry of the valvetrain. The sidewards force on the valve stem is increased as RR increases. There may be interference on the PR side, for another. Going higher may cause piston to valve issues. And so on.

But the bottom line is that going from 1.5 to 1.6:1 rockers is almost always a winner on a SBC.

Rich

 

Thanks guys, I'll stick with 1.6 rockers.



-Mike

 

If anything just drop the lift on the exhausts. You don't always extend spring life with that, but you probably don't need the 1.6's on the exhaust. 8 rockers is cheaper than 16 and it would be a good thing to play with on your setup at the track or on the dyno.

Bret

 

Bret, I'm currently running self-aligning rockers and want to switch to NSA's. So, I will need to buy all 16. Do, you know if there is anyplace I can buy a split set?

 

Oh, don't know if this makes a difference. But, the cam has fairly short duration for the lift 224/230 @ .050.

 

Yeah you can buy a split set, that's not a issue. If you can't find a place drop me a line.

Bret